I have this gnuplot script:
set title font "Monospaced,13" 'Add at random spot'
set grid
set key height 2 box left top
set style line 1 \
linecolor rgb '#ff8844' \
linetype 1 linewidth 3 \
pointtype 5 pointsize 1.5
set style line 2 \
linecolor rgb '#22ff22' \
linetype 1 linewidth 3 \
pointtype 5 pointsize 1.5
set xlabel 'Size'
set ylabel 'Microseconds'
set xrange [0:1000000]
plot 'SO.dat' index 1 with linespoints linestyle 1 title "Indexed list", \
'' index 3 with linespoints linestyle 2 title "Tree list
set terminal png size 650,350 enhanced font "Monospaced,13"
set output 'SO.png'
replot
exit
(data is here),
... and it produces:
What I wish to achieve is to tweak the color of the legend box. How could I do it?

